Notes on the Breeding Behaviour and Ecology of Edible Bullfrogs Pyxicephalus Edulis Peters, 1854 in the Limpopo Province, South Africa
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Herpetology Notes, volume 8: 365-369 (2015) (published online on 30 June 2015) Notes on the breeding behaviour and ecology of Edible Bullfrogs Pyxicephalus edulis Peters, 1854 in the Limpopo Province, South Africa Derek Engelbrecht*, Mmatjie Mashao and Ali Halajian The genus Pyxicephalus currently comprises four deep water (300 mm), no inter-male aggression, and species distributed throughout sub-Saharan Africa: amplexing pairs floating and laying eggs like typical the Giant African Bullfrog (Pyxicephalus adspersus Rana [Afrana] species (Channing et al., 1994). These Tschudi, 1838), Narrow-headed Bullfrog (P. angusticeps observations were made near Beira, Mozambique. Parry, 1982), Edible Bullfrog (P. edulis Peters, 1854) Braack and Maguire (2005) observed P. edulis breeding and Calabresi’s Bullfrog (P. obbianus Calabresi, 1927) in the lowveld of South Africa’s Limpopo Province. (Scott et al., 2013). With the exception of the latter, the Their observations could not have been more different other three species have been at the centre of taxonomic from those reported by Channing et al. (1994). Firstly, debate for many years and their species boundaries Braack and Maguire (2005) observed breeding during and distribution are poorly known, with some authors the day following heavy overnight rain (72 and 188 suggesting introgression is occuring in some areas mm respectively). Secondly, they reported males (Poynton, 1964; Parry, 1982; Poynton and Broadley, aggregating in shallow water (<10 cm) at several leks, 1985; Channing et al., 1994). each with a dominant male which is in contrast to the According to Channing et al. (1994) and Channing isolated males calling from deep water as reported by (2001), P. adspersus and P. edulis occur sympatrically Channing et al. (1994). Thirdly, the lowveld males in places, but this notion was disputed by Scott et al. were extremely aggressive to other males and charging, (2013) who maintained that P. adspersus is an inland tossing and biting was common, sometimes resulting species and does not occur on the Mozambique coastal in fatalities. This aggressive behaviour, albeit more plain. Although P. edulis and P. angusticeps occur muted compared to P. adspersus, was also reported sympatrically on the Mozambique coastal plain, the by Cook and Minter (2004). Finally, amplexus and two species seem to occupy different micro-habitats egg-laying were similar to that of P. adspersus in that (Scott et al., 2013). Pickersgill (2007) evidently found this occurred in shallow water with the female arching these species so confusing that he concluded that her back to expose her cloaca above the water surface, the differences were unfathomable. The taxonomic which is different to the floating, Afrana-[Rana] like confusion has led to seemingly conflicting reports amplexus described by Channing et al. (1994). It is regarding the breeding biology of the Edible Bullfrog. clear from the foregoing that there are contrasting The first description of the breeding biology of P. breeding strategies in Pyxicephalus. Scott et al. (2013) edulis revealed a species that breeds at night after suggested that the observations reported by Channing et light rain with well-spaced males calling in relatively al. (1994) were actually of P. angusticeps and those by Braack and Maguire (2005) of P. edulis. It is clear that more details are required about the breeding biology of these species to resolve the uncertainty regarding the seemingly conflicting breeding strategies of the smaller Pyxicephalus species in southern Africa. Department of Biodiversity, University of Limpopo, Private Recording Pyxicephalus breeding behaviour is not Bag X1106, Sovenga, 0727, South Africa. easy and therefore most of our present knowledge of * Corresponding author e-mail: [email protected] P. edulis behaviour is based on anecdotal observations. 366 Derek Engelbrecht et al. Figure 1. Aggressive behaviour of adult male Edible Bullfrogs (Pyxicephalus edulis). (A) A lunge to knock a rival over; (B) Attack with the mouth open to bite a rival. The species spends approximately 10 months of the year Both events included several hundred frogs, and with below the soil surface (Mitchell, 1946) and only seems the January 2013 breeding event in particular, males to breed in numbers if sufficiently heavy rain (>70 mm) were seen and heard calling on the inundated floodplain fell within a short period (Braack and Maguire, 2005; of an earth dam and the adjacent flooded grassland DE, pers. obs.). Following the main breeding event, (0.2–0.3 km2) as well as many small, temporary pools calling is sporadic and it can be difficult to determine if at various localities all over the farm. These temporary the species is present in a given area. pools included roadside ditches, shallow excavations Here we report on observations made during two and small, natural depressions ranging in size from breeding events of P. edulis, with notes on predation approximately 1 m2 to >100 m2. They were all shallow and the diet of adults in the Limpopo Province, South (< 200 mm) and characterised by flooded grasses and Africa. Specimens were identified based on the forbs. Since there were no good follow-up rains, these presence of a pale spot on the tympanum, the vertebral temporary pools all dried up within 2–3 days and it was stripe in the adults, their distinctive advertisement only at the larger floodplain and inundated grassland calls and the key provided by Scott et al. (2013). Both area where juveniles were later recorded. Although breeding events occurred on the farm De Loskop some swarms of tadpoles were observed, we found (23°30.706’S, 29°19.019’E; 1090 masl), approximately no evidence of channel construction or guarding of 45 km north of the city of Polokwane. The farm is tadpoles. Admittedly our visits to the area were brief situated in the savannah biome and the vegetation is and sporadic after the breeding event and it is possible described as Polokwane Plateau Bushveld (Mucina and that we may have missed such behaviour. It should be Rutherford, 2006). The first breeding event occurred noted that channel construction on the floodplain of the on the 4th December 2009 following heavy rain (>80 earth dam and the adjacent grassland would probably mm) the night before, and the second occurred on not have been required as the area remained flooded for the 21st January 2013 after in excess of 110 mm rain several weeks as a result of a slow but steady inflow fell in the late afternoon and night of the 20th January into the dam. On both occasions breeding took place 2013, resulting in extensive flooding of grassy plains. during the day and by noon breeding activity and calling Notes on the breeding behaviour and ecology of Edible Bullfrogs in South Africa 367 Males called in shallow water with their heads and vocal sacs above the water, their deep calls causing a rippling effect in the water (Figure 4). A spectrogram of a series of three calls is presented in Figure 5. In addition to an auditory clue provided by calling males, the pale bulging vocal sac contrasting with the bright yellow axillary and ventrolateral regions may also provide valuable clues about the breeding status of an individual as suggested by Oliver and Scott-Prendini (2011). It would be interesting to establish if females and rival males use the ripples as sensory cues to determine a male’s fitness like in the Neotropical Túngara Frog (Halfwerk et al., 2014). As mentioned above, there was a marked decrease in calling as the day progressed and we heard no calling frogs when we returned three days later. To obtain data on the diet of adults, we collected the carcasses of five adults that were killed when crossing Figure 2. A male Edible Bullfrog (Pyxicephalus edulis) a road near the breeding site and which were still in attempting amplexus with a corpse. relatively good condition. The stomachs were cut open and the contents sorted in a petri dish and examined under a stereo-microscope in the laboratory. The following prey items were recorded and the percentage of stomachs that contained the items is given in parentheses: Coleoptera (100%), Orthoptera (100%), had decreased substantially. Despite an intensive night- time survey on the 24th January 2013 during favourable weather conditions, no adults were heard or seen in the area. Our observations confirm the observations of Braack and Maguire (2005). Males aggregated in loose groups in shallow water, with an apparently dominant male charging, biting and even tossing other males that got too close (Figures 1A and B). However, males sometimes wandered into a territory and headed straight for the dominant male. Aggressive encounters frequently resulted in some serious injuries and even a few fatalities. Interestingly, one male was observed amplexing a corpse for several minutes (Figure 2). Females seemed to move freely within and between these aggregations with males chasing them. The events immediately preceding amplexus was uncomplicated and oviposition commenced immediately after amplexus. Amplexing pairs were generally not disturbed by others, but amplexus displacement was observed on two occasions. Some females produced notably smaller clutches suggesting that they had spawned earlier already. Spawning occurred in shallow water (Figure 3) and was as described by Braack and Maguire (2005) Figure 3. Amplexus in the Edible Bullfrog (Pyxicephalus and is like that of P. adspersus (Channing et al., 1994; edulis). Note the raised cloaca of the female above the surface Du Preez and Cook 2004). of the water. 368 Derek Engelbrecht et al. Figure 5. A spectrogram of the calls of the Edible Bullfrog (Pyxicephalus edulis) recorded at a breeding event on the farm De Loskop, Limpopo Province, South Africa on 21 January 2013. Figure 4. A calling male Edible Bullfrog (Pyxicephalus metamorphosed African Bullfrogs (Conradie et al., edulis) does not only provide an auditory clue, but also visual 2010).